Add a section on tracking bundle size

content/blog/2017-12-15-improving-the-repository-infrastructure.md

@@ -322,6 +322,20 @@ This works by essentially forcing Flow to verify that two types are assignable t
 
 To conclude this section, it is important to note that you can't specify your own module forks if you consume React from npm. This is intentional because none of these files are public API, and they are not covered by the [semver](https://semver.org/) guarantees. However, you are always welcome to build React from master or even fork it if you don't mind the instability and the risk of divergence. We hope that this writeup was still helpful in documenting one possible approach to targeting different environments from a single JavaScript library.
 
+### Tracking Bundle Size
+
+As a final build step, we now [record build sizes for all bundles](https://github.com/facebook/react/blob/d906de7f602df810c38aa622c83023228b047db6/scripts/rollup/build.js#L264-L272) and write them to a file that [looks like this](https://github.com/facebook/react/blob/d906de7f602df810c38aa622c83023228b047db6/scripts/rollup/results.json). When you run `yarn build`, it prints a table with the results:
+
+<br>
+
+<img src="https://user-images.githubusercontent.com/1519870/28427900-80487dbc-6d6f-11e7-828d-1b594bd1ddb5.png" style="max-width:100%" alt="Build results after running GCC">
+
+*(It doesn't always look as good as this. This was the commit that migrated React from Uglify to Google Closure Compiler.)*
+
+Keeping the file sizes committed for everyone to see was helpful for tracking size changes and motivating people to find optimization opportunities.
+
+We haven't been entirely happy with this strategy because the JSON file often causes merge conflicts on larger branches. Updating it is also not currently enforced so it gets out of date. In the future, we're considering integrating a bot that would comment on pull requests with the size changes.
+
 ## In Conclusion
 
 Did this post inspire you to try some of these ideas in your own projects? We certainly hope so! If you have other ideas about how React build, test, or contribution workflow could be improved, please let us know on [our issue tracker](https://github.com/facebook/react/issues).